1. Field of the Invention
The present invention relates to a active material for positive electrode for an alkaline secondary cell and a method for producing the same as well as an alkaline secondary cell using the above active material for positive electrode, especially a nickel-hydrogen secondary cell and a method for producing the same. More particularly, the present invention is concerned with to a active material for positive electrode exhibiting high utilization and a method for producing the same, and an alkaline secondary cell, especially a nickel-hydrogen secondary cell, which is advantageous not only in that the rise in cell internal pressure is suppressed and both the charge-discharge cycle characteristics and the overdischarge characteristics are improved, but also in that it has excellent storage properties.
2. Prior Art
As representative examples of alkaline secondary cells, there can be mentioned a nickel-hydrogen secondary cell and a nickel-cadmium secondary cell. These cells have incorporated thereinto, as a positive electrode, a nickel electrode comprised mainly of nickel hydroxide which is a active material for positive electrode.
As the nickel electrode, one which is of sinter-type and one which is of paste-type have conventionally been used.
Of these, the sinter-type positive electrode is generally produced as follows. For example, there can be mentioned a method in which nickel particles are sintered in a two-dimensional substrate comprised of a perforated steel or a nickel net to prepare a porous substrate, and the pores in the sinter of the above nickel particles in the prepared porous substrate is impregnated with an aqueous solution of a nickel salt, and further, the aqueous solution of a nickel salt is converted into nickel hydroxide which is a active material for positive electrode using an alkaline aqueous solution.
However, in the above production method, cumbersome treatments are needed, such as an impregnation of an aqueous solution of a nickel salt, a treatment using an alkaline aqueous solution and the like. Further, for forming a active material for positive electrode (nickel hydroxide) in the predetermined amount, it is required to repeat the above-mentioned treatments 4 to 10 times, so that a problem arises in that the production cost for a positive electrode is increased.
In addition, when the porosity of the sinter of nickel particles which are sintered in the above porous substrate exceeds 80%, it is difficult to secure the mechanical strength of the sinter. Therefore, the porosity of the sinter cannot be increased to 80% or more, resulting in a restriction on the method for increasing the capacity of the positive electrode by increasing the amount of the active material added.
For solving the above problems, studies have been made on the paste-type positive electrode produced by a method in which nickel hydroxide particles, particles of a conductor and a binder are kneaded together with water, to thereby prepare a paste for an active material, and a conductive core material (current collector) having a three-dimensional network structure, such as a spongy porous metal or a metal mat fiber, is filled with the above prepared paste, and then, dried and subjected to calendering treatment successively, and the paste-type positive electrode is being put into practical use.
This paste-type positive electrode has a porosity and an average pore size of the conductive core material larger than those in the above-mentioned sinter-type positive electrode. Therefore, the filling of the core material with the active material paste, i.e., active material, is easy, and further, the amount of the active material added can be increased. Thus, from the viewpoint of increasing the capacity of a cell, the paste-type positive electrode is advantageous, as compared to the sinter-type positive electrode.
However, on the other hand, the paste-type positive electrode has the following problem. Specifically, since the pore size of the conductive core material is large, the distance between the active material in the pores of the core material and the skeleton of the core material (current collector passage) is large. In addition, the active material per se is non-conductive. Therefore, the conductivity of the positive electrode itself becomes poor, and the active material at a position far from the skeleton of the core material does not relate to the cell reaction, so that a problem is encountered that the utilization of the active material is lowered.
For solving the above problem, with respect to the paste-type positive electrode, a number of studies have been made to increase the utilization of the active material by satisfactorily securing the electrical connection state between the active materials or the active materials and the conductive core material.
The most common method for solving the problem is a method in which, during the preparation of the positive electrode paste, particles of metallic Co or a cobalt (Co) compound, such as a Co hydroxide or a Co oxide, is added and mixed in the predetermined amount as a conductive auxiliary.
When a positive electrode having a core material filled with the above-mentioned positive electrode pate is incorporated into an alkaline secondary cell, any metallic Co or any Co compound contained in the positive electrode pate is dissolved in the alkaline electrolytic liquid as complex ions once, and the ions are distributed on the surfaces of the nickel hydroxide particles. Then, when the cell is subjected to initial charging, these complex ions are oxidized prior to nickel hydroxide and converted into cobalt oxyhydroxide which is conductive. The resultant cobalt oxyhydroxide is deposited between the nickel hydroxide particles which are an active material or between the active material layer and the skeleton of the conductive core material, to thereby form a conductive matrix in the positive electrode. As a result, both the conductivity between the nickel hydroxide particles as an active material and the conductivity between the active material and the conductive core material are improved, so that the utilization of the active material in the positive electrode is improved.
In such a case, the above-mentioned metallic Co and Co compound are unstable in air, and it is difficult to uniformly mix these particles with the nickel hydroxide particles. Therefore, for increasing the utilization of nickel hydroxide while securing the conductivity as a positive electrode, it is necessary that the amount of the Co compound added be about 10% by mass.
However, when the amount of the Co compound added is large, the relative content of the nickel hydroxide particles (active material) in the active material paste prepared is reduced. In addition, from the viewpoint of the cell design, there is a need to form a discharge reserve for the counter electrode (negative electrode), and this prevents the capacity of the positive electrode from being increased.
The positive electrode is generally accommodated in a cell casing together with a negative electrode, a separator and an alkaline electrolytic liquid.
Then, the assembled cell is subjected to aging treatment and initial charging treatment so as to subject the active material for positive electrode incorporated to activation treatment.
In this case, since the Co compound coexisting with the active material (nickel hydroxide particles) in the positive electrode is electrochemically reversible, the Co compound is discharged during standing of the cell for a long term or due to a microleak current by an electronic switch when the cell is mounted into an appliance, and loses the conductivity thereof. For this reason, although varies depending on the environmental conditions, the cell capacity is sometimes lowered by about 10 to 20% of the rated capacity.
In addition to the above-mentioned method for solving the problem, there is also a method in which, prior to the preparation of an active material paste, the surface of the active material (nickel hydroxide particles) to be used is preliminarily coated with a Co oxide to impart a surface conductivity to the active material, and an active material paste is prepared using the resultant active material.
For example, Japanese Unexamined Patent Publication No. Hei 8-148145 discloses a method in which a bivalent Co compound is disposed on the surface of nickel hydroxide particles, and subjected to heat treatment in the presence of an alkaline aqueous solution and oxygen (air). By this method, the crystalline structure of the Co compound becomes irregular, and the above Co compound is converted into an oxide of Co having a valence of more than 2, specifically, an oxide of Co having a valence of 2.9, to thereby produce an active material comprising composite nickel hydroxide particles in which the nickel hydroxide particles are coated with a matrix of the converted Co oxide. Further, this prior art document discloses the characteristics of a cell having incorporated thereinto a nickel electrode using the active material comprising the above composite nickel hydroxide particles.
The cell having incorporated thereinto a positive electrode using an active material containing an oxide of Co having a valence of about 2.9, that is, Co which is almost completely oxidized, is improved in the capacity recovery ratio after short-circuiting standing (capacity recovery ratio after overdischarging); however, the utilization of the active material is lower than that of the conventional active material containing an oxide of Co having a valence of about 2. In addition, there is a problem in that the cell internal pressure rises during the charging.
This phenomenon is caused by the fact that, when Co is oxidized to an extent such that the valence thereof becomes about 2.9, the resultant Co oxide is extremely stable in the alkaline electrolytic liquid in the cell and is not dissolved in the alkaline electrolytic liquid, and thus, reformation of the above-mentioned conductive matrix bonding together the composite nickel hydroxide particles hardly occurs. Further, it is considered that the above phenomenon is caused by the fact that a small amount of Co transferred to the surface of the negative electrode is remarkably reduced, and hence, the gas absorbing power at the negative electrode becomes poor.
In addition, Japanese Unexamined Patent Publication No. Hei 9-213326 discloses a method in which nickel hydroxide particles are subjected to alkali heat treatment by adding to the nickel hydroxide particles an alkaline aqueous solution, such as an aqueous sodium hydroxide solution, and an aqueous solution of a Co compound, such as cobalt sulfate, simultaneously while heating the nickel hydroxide particles in the presence of oxygen, and thoroughly stirring and mixing the resultant mixture, to thereby coat the surface of the nickel hydroxide particles with a higher oxide of Co containing an alkali component.
However, in this method, actually, before the Co compound added is converted into a higher oxide to coat the surface of the nickel hydroxide particles, an oxidation of the Co compound proceeds and the Co compound is often converted into an oxide having a poor conductivity, so that a satisfactory amount of a conductive matrix is not formed between the nickel hydroxide particles.
Further, Japanese Unexamined Patent Publication No. Hei 9-73900 discloses the following method.
In this method, first, nickel hydroxide particles each having a surface on which cobalt hydroxide is deposited are produced separately. Then, the nickel hydroxide particles are subjected to alkali heat treatment by spraying an alkaline aqueous solution against the nickel hydroxide particles in a fluidization drying apparatus in an open system while flowing hot air through the apparatus, and fluidizing and mixing the entire content of the apparatus, to thereby convert cobalt hydroxide on the surface into a higher oxide of Co.
This method has an advantage in that an occurrence of agglomeration of the particles during the oxidizing treatment for the Co compound can be suppressed; however, this method is one in which a fluidization oxidization is conducted by supplying and evacuating heated air, and thus, it is difficult to obtain a good balance of the heat transmission between the components constituting the reaction system, resulting in a problem that it is extremely difficult to appropriately control the degree of oxidation of the Co compound.
Specifically, the fluidization mixing of the nickel hydroxide particles having the Co compound with the alkaline aqueous solution is conducted in a hot air convection mode using heated air. Therefore, when the concentration of the alkaline aqueous solution is increased to 35% by mass or higher or the amount of the alkaline aqueous solution sprayed per unit time is increased for keeping constant the oxidation reaction time, the fluidization of the nickel hydroxide particles does not occur, so that the fluidization mixing of the nickel hydroxide particles with the alkaline aqueous solution does not proceed.
Further, since air is used as a heat transmission medium, the deviation of heat distribution in the reaction system is large, so that the oxidation reaction of the Co compound ununiformly proceeds.
Actually, in the nickel-hydrogen secondary cell assembled from the positive electrode using the nickel hydroxide particles produced by this method, the utilization of the active material is not so high, and further, the cell internal pressure rises. In addition, the capacity recovery ratio after the cell is stored in a high temperature environment or stored at low temperatures for a long term is lowered, and hence, the storage properties are not excellent.
It is an object of the present invention to provide a active material for positive electrode for an alkaline secondary cell, which is advantageous in that it exhibits high utilization and is useful as an active material for a paste-type positive electrode, and a method for producing the same.
It is another object of the present invention to provide an alkaline secondary cell, especially a nickel-hydrogen secondary cell, which is advantageous not only in that the rise in cell internal pressure is suppressed and the capacity recovery ratio is excellent, but also in that it has excellent overdischarge characteristics and excellent storage properties as well as excellent charge-discharge cycle characteristics, and a method for producing the same.
For attaining the above objects, in the present invention, there is provided:
a active material for positive electrode for an alkaline secondary cell, comprising nickel hydroxide particles each having a surface to which a cobalt oxide sticks,
wherein a part of the cobalt oxide comprises an oxide of cobalt(II).
Specifically, there is provided the active material for positive electrode wherein the oxide of cobalt(II) is present in an amount of 20 to 40% by mole, based on the total mole of the cobalt oxide.
Further, in the present invention, there is provided a method for producing a active material for positive electrode for an alkaline secondary cell, the method comprising:
mixing and stirring nickel hydroxide particles each having a surface to which a cobalt(II) compound sticks or/and mixture particles of nickel hydroxide particles and particles of metallic cobalt or a cobalt(II) compound with an alkaline aqueous solution in the presence of oxygen; and
subjecting the resultant mixed and stirred system to heat treatment by radiation while mixing and stirring.
Still further, in the present invention, there is provided:
an alkaline secondary cell having incorporated thereinto a positive electrode having the above-mentioned active material for positive electrode, and specifically, there is provided a nickel-hydrogen secondary cell, comprising:
a cell casing;
an alkaline electrolytic liquid; and
an electrode group comprising a positive electrode which comprises a conductive core material having carried thereon an active material comprised mainly of nickel hydroxide particles each having a surface coated with a sodium-containing cobalt oxide, or an active material comprised mainly of a mixture of nickel hydroxide particles each having a surface coated with a sodium-containing cobalt oxide and metallic cobalt or a cobalt(II) compound, a negative electrode which comprises a conductive core material having carried thereon a negative electrode preparation comprised mainly of a metal alloy having hydrogen occluded therein, and a separator disposed between the positive electrode and the negative electrode,
wherein the electrode group is sealed in the cell casing together with the alkaline electrolytic liquid,
wherein the alkaline electrolytic liquid contains 0.3 to 1.2 mole of lithium hydroxide,
and there is provided a method for producing the above-mentioned nickel-hydrogen secondary cell, wherein an alkaline electrolytic liquid is poured into a cell casing and the cell casing is sealed up, and then, the resultant casing is allowed to stand in an environment at a temperature of 40 to 100xc2x0 C. for 1 day or more.